Familial adenomatous polyposis, attenuated

Last author update: 19 February 2021
Last staff update: 5 March 2021

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PubMed Search: "Familial adenomatous polyposis" attenuated AFAP

Jennifer Findeis-Hosey, M.D.
Raul S. Gonzalez, M.D.
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Cite this page: Findeis-Hosey J, Gonzalez RS. Familial adenomatous polyposis, attenuated. website. Accessed December 7th, 2022.
Definition / general
  • Subtype of familial adenomatous polyposis (FAP) characterized by fewer than 100 adenomatous colorectal polyps
  • Colonic adenomatous polyps have high risk for progression to colorectal adenocarcinoma (69% cumulative risk by age 80) (Gastroenterology 2004;127:444)
Essential features
  • FAP variant characterized by markedly fewer polyps
  • Due to a defect in APC (5q21)
  • Patients might not require colectomy
  • Phenotypically and genetically heterogeneous (Gut 2006;55:1440)
  • Cancers usually develop at age 50 - 55 years, 15 years later than classic FAP
  • Patients may develop gastric fundic gland polyps, gastric or small bowel flat adenomas and gastric / duodenal carcinomas (Cancer 1993;71:2709)
  • Extracolonic manifestations of classical FAP are less common in attenuated FAP
  • Classically reserved for patients with fewer than 100 colonic adenomatous polyps but exact diagnostic criteria have not been firmly established
Case reports
  • Patients with fewer than 20 - 30 polyps may not require total colectomy if they have frequent surveillance colonscopies with polypectomies
Gross description
  • Polyps are usually more proximal (i.e. right sided) than in classic FAP
    • Rectum often spared
  • Polyps are flat, slightly raised or plaque-like
  • May have minute central depression or umbilication
Microscopic (histologic) description
  • Adenomatous polyps are microscopically similar to sporadic type adenomas
Molecular / cytogenetics description
  • Associated with pathogenic variants in the 5' and distal 3' end of APC, as well as interstitial deletions of 5q22, which include the APC gene

Attenuated familial adenomatous polyposis (FAP)

Differential diagnosis
Board review style question #1
How is attenuated familial adenomatous polyposis different from classic familial adenomatous polyposis?

  1. Patients develop carcinomas at a younger age
  2. The adenomas are fewer in number
  3. The adenomas are smaller in size
  4. The adenomas occur in the small intestine, not the colon
Board review style answer #1
B. The adenomas are fewer in number

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